OPTICAL EVIDENCE OF CHLOROPLAST STRUCTURE 1741 



under the electron microscope, but that their thickness is such that, in 

 sedimenting edgewise, they have a cross-section similar to that of a 0.04 n 

 sphere. For a diameter of 0. 1 1 m, this thickness is 0.05 m- These dimensions 

 are small, but not so small as to make implausible the assumption of func- 

 tional identity of the bacterial "chromatophores" with the grana of the 

 higher plants and algae. More recently grana of approximately this size 

 have been in fact found in bacteria by electron microscopy (Thomas 1952; 

 cf. section 2 above). 



Schachman et al. estimated that a single bacterial cell contains about 

 5,000 colored particles; but this estimate involved several rather uncertain 

 premises. 



In a suspension prepared by grinding blue-green algae, fractional ultra- 

 centrifugation also showed the presence of a single chlorophyll-bearing 

 fraction. The colored particles were even larger than in bacteria. Electron 

 microscopy indicates the presence in blue-green algae of grana of the same, 

 or even somewhat larger, size than in higher plants — about 0.8 n across 

 (Vatter 1952, Thomas 1952). The large-particle fraction of blue-green 

 algae contained all chlorophyll, but phycocyanin was found mainly in a 

 slow-sedimenting fraction. However, the occurrence of phycobilin-sensi- 

 tized chlorophyll fluorescence in algae {cf. Chapter 24), argues against spatial 

 separation of the two pigments in the living chromatoplasm, and for this 

 separation having been effected in the grinding of the cells. This is sup- 

 ported by the observations of McClendon (1952, 1954), cf. section 6 below. 



4. Optical Evidence of Chloroplast Structure 



In Volume 1 (p. 365), measurements of double refraction and dichroism 

 of chloroplasts were described. Frey-Wyssling's (1938) interpretation of 

 these observations given there, was that the "positive" double refraction 

 of chloroplasts in the natural state is a "morphic" birefringence, caused by 

 lamellar structure, while the "negative" double refraction, found after im- 

 bibition of chloroplasts with glycerol, is an "intrinsic" birefringence caused 

 by the presence in the chloroplasts of an orderly array of long rod-shaped 

 hydrocarbon chains, such as exist in lipides and phospholipides. However, 

 Frey-Wyssling noted that the intrinsic double refraction of chloroplasts is 

 weak, and suggested an imperfect alignment. 



Frey-Wyssling and Steinmann (1948), in a more precise study of the 

 double refraction of Mougeotia chloroplasts, used different fixatives, and 

 imbibed the chloroplasts with varying amounts of different liquids. They 

 obtained in this way a set of hyperbolic curves showing double refraction as 

 a function of refractive index. For all fixating solutions, the hyperbolas 

 had a peak at n=1.58 (which is close to the refractive index of protein). 



